Landfill leachate treatment system and process

Abstract

The landfill leachate treatment system comprises an anaerobic reaction system, an aerobic reaction system, a Fenton-like reaction system and a post-treatment system which are sequentially connected, the anaerobic reaction system is further connected with a pre-treatment system, and a sedimentation tank and a pH adjusting tank are further arranged between the aerobic reaction system and the Fenton-like reaction system. And the post-treatment system is a curing system or a water outlet system. And almost all pollutants can be timely and efficiently stabilized, the operation is simple, subsequent products can be used in the building industry, and waste utilization is realized.

Description

Technical Field

[0001] The present invention relates to the technical field of environmental protection, and particularly relates to a garbage leachate treatment system and process. Background Art

[0002] Garbage leachate is an organic wastewater rich in heavy metals and ammonia nitrogen discharged from urban landfills. If not properly disposed of, it will cause serious environmental hazards to the surrounding water, soil and atmospheric environment. With the rapid development of urbanization in China, the output of urban domestic waste has increased sharply, which has led to an increasing discharge of a large amount of garbage leachate with complex components and serious environmental hazards, exacerbating ecological environmental pollution. At present, the treatment technologies for garbage leachate in China include physical and chemical methods, biological methods and combined treatment methods, aiming to make the garbage leachate reach the discharge concentration limit specified in GB16889-2008, and then the treated leachate can be sent to the urban sewage treatment plant for treatment.

[0003] Advanced oxidation technology has a good treatment effect on organic substances in garbage leachate, but the treatment cost is relatively high and the sludge production is large. Moreover, the components of garbage leachate are complex. In order to achieve the best operating conditions required for advanced oxidation treatment, certain pretreatment is also required. In addition, advanced oxidation technology cannot denitrify and reduce the heavy metal concentration.

[0004] Membrane separation technology for advanced treatment can efficiently remove pollutants such as COD, ammonia nitrogen and nitrate nitrogen, but the cost is high and the maintenance is inconvenient.

[0005] Single treatment technology has a poor treatment effect on garbage leachate with complex components. The mainstream solution to effectively solve the pollution of garbage leachate is to combine multiple technologies.

[0006] However, during the traditional treatment of garbage leachate, the subsequent treatment of ammonia nitrogen content is often achieved by adding carbon sources, denitrification and other technologies. Adding carbon sources often requires adding reagents, and there will still be a certain concentration of COD remaining after denitrification treatment, which cannot guarantee the effluent quality. Summary of the Invention

[0007] The purpose of the present invention is to provide a garbage leachate treatment system and process to solve the problems of poor treatment effect of single technology and the remaining certain concentration of COD during ammonia nitrogen removal in the prior art.

[0008] To achieve the above purpose, the present invention provides the following technical solutions: A landfill leachate treatment system provided by the present invention includes an anaerobic reaction system, an anoxic-aerobic reaction system, a Fenton-like reaction system, and a post-treatment system connected in sequence. The anaerobic reaction system is further connected to a pretreatment system. A sedimentation tank and a pH adjustment tank are also provided between the aerobic reaction system and the Fenton-like reaction system. The post-treatment system is a solidification system or an effluent system.

[0009] Further, the pretreatment system includes an oil separation device and a homogenization and adjustment tank.

[0010] Furthermore, the oil separation device is a high-efficiency, non-powered oil-water separation device that can recycle more than 90% of the waste oil, which can not only effectively protect the environment but also greatly save the operation cost. The oil separation device is applicable to the oil-water separation of oily water containing various mineral oils (except crude oil), vegetable oils, animal oils or their mixtures. The principle is a non-powered oil-water separation device. When the oily sewage flows into the screen basket, the screen completely intercepts the solid debris (such as vegetable leaves and food residues) in it; then, using the kinetic energy of the water flow, continuous collisions occur, growing from small to large, thereby accelerating the movement to separate and layer oils with different specific gravities;; finally, the oil and water are separated using the density difference, the clear water flows out from the overflow weir, and is then collected and discharged through the outlet pipe. The floating oil enters the oil collecting pipe and is collected and discharged.

[0011] Further, the anaerobic reaction system is an EGSB high-efficiency anaerobic reaction tower, the anoxic-aerobic reaction system is a three-stage A / O biochemical pool I, and the Fenton-like reaction system is a two-stage high-efficiency heterogeneous Fenton-like reaction tower.

[0012] Furthermore, the EGSB high-efficiency anaerobic reaction tower: (1) EGSB anaerobic reaction mechanism The anaerobic process is divided into four stages: hydrolysis, acidification, acetic acid production, and methane production. In the hydrolysis stage, macromolecular organic substances are decomposed by extracellular enzymes of bacteria into small molecules that can dissolve in water and penetrate the cell membrane; in the acidification stage, the hydrolyzed small molecules are converted into simpler compounds in the cells of acidifying bacteria and secreted outside the cells; in the acetic acid production stage, the products of the hydrolysis and acidification stage are further converted by acetic acid-producing bacteria into acetic acid, hydrogen, carbon dioxide, and new cell substances; in the methanation stage, acetic acid, hydrogen, carbonic acid, formic acid, methanol, etc. produced in the acetic acid production stage are converted into methane, carbon dioxide, and new cell substances.

[0013] Under anaerobic conditions, acidification and corruption reactions occur, degrading macromolecular substances in the sewage into small molecules and converting refractory substances into easily degradable substances, while generating a large amount of biogas.

[0014] (2) Cultivation of sludge in the composite anaerobic reactor In a composite anaerobic reactor, the degree of sludge granulation and activity are the keys affecting the efficiency of composite anaerobic treatment. The cultivation of activated sludge granulation mainly starts from the following aspects: Ⅰ: Strictly control the composition and nutrients of the sewage entering the composite anaerobic reactor (appropriate nutrient elements C:N:P = 200:5:1, control the concentration of toxic substances, maintain pH = 6.5 - 7.5, and supplement appropriate trace elements: Ca 2+ 、Fe 3+ ).

[0015] Ⅱ: Accurately design the structure and parameters of the reactor. The three-phase separator, which is the main component in composite anaerobic, has good hydraulic mixing and three-phase separation characteristics, thus cultivating granulated activated sludge with high activity and good sedimentation performance.

[0016] Ⅲ: The composite anaerobic influent adopts a multi-pipe and multi-point water distribution system to evenly distribute the influent at the bottom of the reactor and use an appropriate upward flow velocity to wash out the fine and dispersed sludge in the reactor, ensuring that the influent volume per unit area is basically the same, which is conducive to the completion of granulation and thus avoids the occurrence of dead sludge accumulation caused by dead corners.

[0017] Ⅳ: During the start-up process of the composite anaerobic reactor, add mature sludge from other projects + highly efficient anaerobic strains (or other bacterial agents) to quickly debug and start. In the initial stage of start-up, the leachate enters the treatment system in proportion and the load is increased in a timely manner, so that the microorganisms can obtain sufficient nutrients and better form granulated sludge.

[0018] Ⅴ: While carrying out hydraulic agitation in the sludge bed, fully consider the influence of hydraulic agitation and biogas agitation on the mixing of influent and sludge to prevent local acidification.

[0019] Ⅵ: The EGSB anaerobic reactor adopts anaerobic granular sludge technology, and this technology has certain requirements for temperature. The appropriate medium-temperature range helps the formation of granular sludge and maintains the stability of the system. The control of temperature has an important impact on the start-up and stability of the EGSB reactor. The EGSB reactor can be successfully started and form granular sludge under normal temperature, medium temperature and high temperature conditions, but the vast majority of research is carried out under medium temperature conditions. This is because medium temperature conditions are conducive to the activities of anaerobic microorganisms, thus improving the treatment efficiency. To ensure the treatment effect, the anaerobic reactor is heat-insulated according to the local climate conditions.

[0020] Furthermore, the three-stage A / O biochemical pool: The fixed-bed multi-stage composite enhanced aerobic process adopts the natural INSR enhanced - A / O / A / O + secondary sedimentation tank process. The dissolved oxygen in the aerobic section is controlled at about 0.5 to achieve natural enhanced aerobic treatment. Through the treatment of the enhanced A / O process, the purpose of degrading organic matter, denitrifying and removing phosphorus is achieved.

[0021] The three-stage A / O biochemical pool uses the combined A / O process to achieve the effect of three-stage denitrification and biochemical treatment, which can effectively improve the removal rates of indicators such as COD and ammonia nitrogen. The wastewater first passes through the anoxic tank and then enters the aerobic tank, and the mixed liquid of the aerobic tank and the sludge of the sedimentation tank are simultaneously refluxed to the anoxic tank. The A / O process connects the front anoxic section and the rear aerobic section in series. The DO in the A section is not greater than 0.2, and the DO in the O section is 2 - 4. In the anoxic section, heterotrophic bacteria hydrolyze suspended pollutants and soluble organic matters such as starch, fiber, and carbohydrates in the sewage into organic acids, decompose macromolecular organic matters into small-molecular organic matters, and convert insoluble organic matters into soluble organic matters. When these products hydrolyzed in the anoxic section enter the aerobic tank for aerobic treatment, the biodegradability of the sewage can be improved, and the efficiency of oxygen can be enhanced; in the anoxic section, heterotrophic bacteria ammonify pollutants such as proteins and fats (N on the organic chain or amino groups in amino acids), releasing ammonia (NH3, NH4 + ), under sufficient oxygen supply conditions, the nitrification of autotrophic bacteria oxidizes NH3-N (NH4 + to NO3 - , and through reflux control, it returns to the A tank. Under anoxic conditions, the denitrification of heterotrophic bacteria reduces NO3 - to molecular nitrogen (N2), completing the cycle of C, N, and O in the ecosystem and realizing the harmless treatment of sewage The organic carbon in the sewage in the anoxic tank is utilized by denitrifying bacteria, which can reduce the organic load of the subsequent aerobic tank. The alkalinity generated by the denitrification reaction can compensate for the alkalinity demand for the nitrification reaction in the aerobic tank. The aerobic stage after the anoxic tank can further remove the remaining organic pollutants in the denitrification process and improve the effluent quality. The fixed-bed three-stage A / O biochemical pool is inoculated with activated sludge, which is convenient for the rapid startup of the system.

[0022] Furthermore, the two-stage high-efficiency heterogeneous Fenton reaction tower: The operation process is basically the same as the Fenton process.

[0023] The heterogeneous Fenton catalyst (EX-2) is an iron-based catalyst with a micro-nano structure and is a consumable catalyst. This avoids the problems of large consumption of traditional millimeter-scale catalysts and the decline in catalytic effect caused by the inactivation of surface catalytic active sites α-hydroxy iron oxide. On the other hand, the micro-nano scale catalyst can be suspended in water with a small particle size and is in the form of a fluidized bed, avoiding the problem of blockage of active sites and inactivation caused by the impact of oils, SS, etc. on traditional millimeter-scale catalysts. At the same time, due to the small particle size and large specific surface area of this catalyst, the catalyst dosage is significantly reduced compared with traditional millimeter-scale catalysts. The filling amount of traditional catalysts is 30%, while this catalyst only requires 3 - 12 g / L, and the filling amount is 0.3% - 0.6%, a reduction of more than 98%, thus effectively reducing the cost.

[0024] During the operation of the heterogeneous Fenton catalyst, part of it is lost, and 5% of the catalyst needs to be replenished annually. 1.5 tons of catalyst are replenished in the first-level heterogeneous Fenton tower every year, and 1.0 ton of catalyst is replenished in the second-level heterogeneous Fenton tower every year.

[0025] Further, the curing system includes a curing chamber and a drying chamber. The curing chamber is provided with a feed inlet, and the drying chamber is provided with a mold and a temperature control system.

[0026] Further, the water outlet system includes a three-stage A / O biochemical pond II, a coagulation sedimentation tank, a biological activated carbon filter tank, and an ultraviolet disinfection system.

[0027] Furthermore, a three-stage A / O biochemical pond and two-stage high-efficiency heterogeneous Fenton reaction towers can be repeatedly arranged between the water outlet system and the Fenton-like reaction system.

[0028] The present invention also provides a process for a landfill leachate treatment system. When the post-treatment system is a curing system, the treatment process includes the following steps: S1. Pretreatment: After passing through an oil removal device, kitchen waste and landfill leachate enter a homogeneous regulation pond for homogenization; S2. Anaerobic treatment: In an EGSB high-efficiency anaerobic reaction tower, macromolecular substances are degraded into small-molecular substances, and refractory substances are converted into easily degradable substances; S3. Anoxic-aerobic treatment: Part of the COD, ammonia nitrogen, and phosphorus are removed in the first three-stage A / O biochemical pond; S4. Precipitation and pH adjustment: Calcium peroxide, aluminum sulfate, ferric chloride, and polyaluminum chloride are added to the sedimentation tank to remove fulvic acid and humic acid substances, and then the pH is adjusted to 3 in a pH adjustment tank; S5. Fenton-like reaction: A heterogeneous Fenton catalyst is used for the Fenton oxidation process to remove COD; S6. Curing: A curing agent and a regulator are added to the liquid in the curing chamber. After being mixed evenly, an inorganic gel material is added and sent into the drying chamber, and the obtained product is used in the construction industry.

[0029] Further, the fulvic acid removing agent is calcium peroxide, aluminum sulfate, ferric chloride, and polyaluminum chloride.

[0030] Further, the curing agent is steel slag and furnace slag, and the regulator is phosphate and oxalate.

[0031] Further, when the post-treatment system is a water outlet system, S6 is to further remove COD, ammonia nitrogen, and phosphorus in the second three-stage A / O biochemical pond, and further includes: S7. Coagulation sedimentation: A coagulant and a coagulation aid are added to the coagulation sedimentation tank for sedimentation; S8. Adsorption: Activated carbon is used to adsorb organic substances and dissolved oxygen in water; S9, Disinfection and Effluent: After ultraviolet disinfection, it is stored in the clean water tank.

[0032] After the landfill leachate undergoes front-end treatment, the effluent contains some fulvic acid and humic acid substances. Humic acid is the main component of dissolved organic matter in landfill leachate and is also the main reason for the relatively high COD remaining in the leachate after biochemical treatment. Humic acid substances are difficult to effectively biotreat and will also inhibit the biochemical process.

[0033] Using the chemical precipitation method to remove fulvic acid and humic acid substances from the leachate has good effects.

[0034] Based on the above technical solutions, the embodiments of the present invention can at least produce the following technical effects: (1) In the curing system of the present invention, the acidic substances in the liquid react with the metal oxides in the curing agent, and at the same time, metal ions are partially dissolved out, avoiding direct reaction with cement and consuming the alkaline substances in the cement; the addition of the regulator enables ammonia nitrogen to form a metal-ammonia nitrogen complex salt, which is stabilized in the cured body, preventing the release of ammonia gas, and having obvious environmental effects; it can quickly and efficiently stabilize almost all pollutants, and the operation is simple. The subsequent products can be used in the construction industry to realize waste utilization.

[0035] (2) The Fenton-like oxidation technology of the present invention is a technology that uses a solid catalyst to replace Fe in traditional Fenton 2+ to activate H2O2 to generate reactive oxygen species such as ·OH. It makes up for the deficiencies of the classical homogeneous Fenton reaction. By transferring the main pollutant degradation site from the solution homogeneous phase to the solid-liquid interface, it effectively reduces the generation of iron mud and improves the cyclic stability of the degradation system at the same time. Detailed Embodiments

[0036] Now, the various exemplary embodiments of the present invention will be described in detail. This detailed description should not be considered as a limitation of the present invention, but should be understood as a more detailed description of certain aspects, characteristics, and implementation schemes of the present invention.

[0037] It should be understood that the terms described in the present invention are only for describing specific embodiments and are not used to limit the present invention. In addition, for the numerical ranges in the present invention, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Each intermediate value within any stated value or stated range and each smaller range between any other stated value or intermediate value within the stated range are also included in the present invention. The upper and lower limits of these smaller ranges can be independently included or excluded from the range.

[0038] Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. Although this invention only describes preferred methods and materials, any methods and materials similar or equivalent to those described herein can also be used in the implementation or testing of this invention. All documents mentioned in this specification are incorporated by reference to disclose and describe the methods and / or materials related to the said documents. In case of conflict with any incorporated document, the content of this specification shall prevail.

[0039] Without departing from the scope or spirit of this invention, various improvements and changes can be made to the specific embodiments of the specification of this invention, which are obvious to those skilled in the art. Other embodiments obtained from the specification of this invention are obvious to those skilled in the art. The specification and examples of this invention are merely exemplary.

[0040] Regarding the use of "comprising", "including", "having", "containing", etc. herein, they are all open-ended terms, meaning including but not limited to.

[0041] The terms "installed", "connected", "linked" should be understood in a broad sense. For example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection; it can be a direct connection, or an indirect connection through an intermediate medium, and it can be the communication inside two components. For those of ordinary skill in the art, the specific meanings of the above terms in this application can be understood according to specific circumstances.

[0042] Example 1

[0043] A landfill leachate treatment system includes an anaerobic reaction system, an anoxic-aerobic reaction system, a Fenton-like reaction system, and a post-treatment system connected in sequence. The anaerobic reaction system is also connected to a pretreatment system. A sedimentation tank and a pH adjustment tank are also provided between the aerobic reaction system and the Fenton-like reaction system. The post-treatment system is a solidification system or an effluent system. The pretreatment system includes an oil separation device and a homogenization and adjustment tank.

[0044] The solidification system includes a solidification chamber and a drying chamber. The solidification chamber is provided with a feed inlet, and the drying chamber is provided with a mold and a temperature control system.

[0045] A process for a landfill leachate treatment system, the treatment process includes the following steps: S1. Pretreatment: After the kitchen waste passes through the oil separation device, it enters the homogenization and adjustment tank with the landfill leachate for homogenization; S2. Anaerobic treatment: In the EGSB high-efficiency anaerobic reaction tower, macromolecular substances are degraded into small-molecular substances, and refractory substances are converted into easily degradable substances; S3. Anoxic-aerobic treatment: Remove part of COD, ammonia nitrogen, and phosphorus in the first three-stage A / O biochemical pond; S4. Precipitation and pH adjustment: Add calcium peroxide, aluminum sulfate, ferric chloride, and polyaluminum chloride as fulvic acid removers to the sedimentation tank to remove fulvic acid and humic acid substances, and then adjust the pH to 3 in the pH adjustment tank; S5. Fenton-like reaction: Use a heterogeneous Fenton-like catalyst to carry out the Fenton oxidation process to remove COD; S6. Solidification: Add a solidifying agent and a regulator to the liquid in the solidification chamber. Utilize the reaction between the acidic substances in the liquid and the metal oxides in the solidifying agent. At the same time, part of the metal ions dissolve out, avoiding direct reaction with cement and consuming the alkaline substances in the cement. The addition of the regulator enables ammonia nitrogen to form a metal-ammonia nitrogen composite salt and be stabilized in the solidified body, preventing the release of ammonia gas. After mixing evenly, add an inorganic gel material and send it to the drying chamber. The obtained product is used in the construction industry.

[0046] The fulvic acid remover is calcium peroxide, aluminum sulfate, ferric chloride, and polyaluminum chloride.

[0047] The solidifying agent is steel slag and furnace slag, and the regulator is phosphate and oxalate.

[0048] Example 2

[0049] A landfill leachate treatment system includes an anaerobic reaction system, an anoxic-aerobic reaction system, a Fenton-like reaction system, and a post-treatment system connected in sequence. The anaerobic reaction system is also connected to a pretreatment system. A sedimentation tank and a pH adjustment tank are also provided between the aerobic reaction system and the Fenton-like reaction system. The post-treatment system is a solidification system or an effluent system. The pretreatment system includes an oil separation device and a homogenization and adjustment tank.

[0050] The effluent system includes a second three-stage A / O biochemical pond, a coagulation sedimentation tank, a biological activated carbon filter tank, and an ultraviolet disinfection system.

[0051] A process for a landfill leachate treatment system, the treatment process includes the following steps: S1. Pretreatment: After the kitchen waste passes through the oil separation device, it enters the homogenization and adjustment tank together with the landfill leachate for homogenization; S2. Anaerobic treatment: Degrade macromolecular substances into small-molecular substances and convert refractory substances into easily degradable substances in the EGSB high-efficiency anaerobic reaction tower; S3. Anoxic-aerobic treatment: Remove part of COD, ammonia nitrogen, and phosphorus in the first three-stage A / O biochemical pond; S4. Precipitation and pH adjustment: Add calcium peroxide, aluminum sulfate, ferric chloride, and polyaluminum chloride as fulvic acid removers to the sedimentation tank to remove fulvic acid and humic acid substances, and then adjust the pH to 3 in the pH adjustment tank; S5. Fenton-like reaction: Use a heterogeneous Fenton-like catalyst to carry out the Fenton oxidation process to remove COD; S6. Further remove COD, ammonia nitrogen and phosphorus in the second three-stage A / O biochemical pond; S7. Coagulation and precipitation: Add coagulants and coagulant aids in the coagulation sedimentation tank for precipitation; S8. Adsorption: Use activated carbon to adsorb organic substances and dissolved oxygen in water; S9. Disinfection and effluent discharge: After ultraviolet disinfection, store it in the clear water tank.

[0052] The above are only the preferred embodiments of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the inventive concept of the present invention, several modifications and improvements can still be made, and these all belong to the protection scope of the present invention.

Claims

1. A landfill leachate treatment system, characterized in that, It includes an anaerobic reaction system, an anoxic-aerobic reaction system, a Fenton-like reaction system and a post-treatment system connected in sequence. The anaerobic reaction system is also connected with a pretreatment system. A sedimentation tank and a pH adjustment tank are also provided between the aerobic reaction system and the Fenton-like reaction system. The post-treatment system is a solidification system or an effluent system.

2. The landfill leachate treatment system according to claim 1, characterized in that, The pretreatment system includes an oil separation device and a homogenization and adjustment tank.

3. The garbage leachate treatment system according to claim 1, characterized in that, The anaerobic reaction system is an EGSB high-efficiency anaerobic reaction tower, the anoxic-aerobic reaction system is a three-stage A / O biochemical pool I, and the Fenton-like reaction system is a two-stage high-efficiency heterogeneous Fenton-like reaction tower.

4. The leachate treatment system according to claim 1, characterized in that The solidification system includes a solidification chamber and a drying chamber. The solidification chamber is provided with a feed inlet, and the drying chamber is provided with a mold and a temperature control system.

5. The garbage leachate treatment system according to claim 1, wherein, The effluent system includes a three-stage A / O biochemical pool II, a coagulation sedimentation tank, a biological activated carbon filter tank and an ultraviolet disinfection system.

6. The process of the landfill leachate treatment system according to any one of claims 1-5, characterized in that, When the post-treatment system is a solidification system, the treatment process includes the following steps: S1. Pretreatment: After passing through the oil separation device, the food waste and the landfill leachate enter the homogenization and adjustment tank for homogenization. S2. Anaerobic treatment: In the EGSB high-efficiency anaerobic reaction tower, macromolecular substances are degraded into small-molecular substances, and refractory substances are converted into easily degradable substances. S3. Anoxic-aerobic treatment: Part of the COD, ammonia nitrogen and phosphorus are removed in the three-stage A / O biochemical pool I. S4. Sedimentation and pH adjustment: Calcium peroxide, aluminum sulfate, ferric chloride and polyaluminum chloride are added to the sedimentation tank to remove fulvic acid and humic acid substances, and then the pH is adjusted to 3 in the pH adjustment tank. S5. Fenton-like reaction: A heterogeneous Fenton catalyst is used for the Fenton oxidation process to remove COD. S6. Solidification: The curing agent and the regulator are added to the liquid in the solidification chamber. After mixing evenly, an inorganic gel material is added and sent into the drying chamber. The obtained product is used in the construction industry.

7. The process of the landfill leachate treatment system according to claim 6, characterized in that, The fulvic acid remover is calcium peroxide, aluminum sulfate, ferric chloride and polyaluminum chloride.

8. The process of the landfill leachate treatment system according to claim 6, characterized in that, The curing agent is steel slag and furnace slag, and the regulator is phosphate and oxalate.

9. The process of the landfill leachate treatment system according to claim 6, characterized in that, When the post-treatment system is an effluent system, S6 is to further remove COD, ammonia nitrogen and phosphorus in the three-stage A / O biochemical pool II, and it also includes: S7. Coagulation sedimentation: A coagulant and a coagulation aid are added to the coagulation sedimentation tank for sedimentation. S8. Adsorption: Activated carbon is used to adsorb organic substances and dissolved oxygen in the water. S9. Disinfection and effluent: After ultraviolet disinfection, it is stored in the clear water tank.